The fan case of a turbofan engine performs several functions in association with the rotating fan in operation. The aerodynamic function of the fan case is to direct the axial flow of air in conjunction with the fan.
Typically the fan directs a primary air stream through the compressor and turbines of the engine and secondary airflow through an annular radially outward bypass duct. For the aerodynamic function of the fan case, it is essential that the clearance between the rotating fan blades and the internal surface of the fan case be kept within an acceptable range to maximize the fan efficiency.
It is common practice to line the internal air path surfaces of the fan case with an abradable material. On initial operation of the engine and rotation of the newly manufactured fan, the abradable material is rubbed off on contact with the tips of the rotating fan blade. For example, in the case of small gas turbine engines with a fan of diameter approximately 24 inches, the thickness of the abradable layer of material is in the order of 0.070 inches. At assembly conditions, the tip clearance is in the order of 0.005 to 0.030 inches. During the high speed rotation of the fan, the fan blades stretch elastically under the load of centrifugal force in the order of 0.020 to 0.040 inches. Due to the dynamic stretching of the metallic blades, the abradable material is abraded on contact with the fan blade tips. Due to manufacturing tolerances, each fan blade will have its unique variation and the actual degree of running clearance required and stretching of blades will vary a certain amount between different fans when manufactured. The provision of abradable material therefore allows for close tolerance or minimizing of clearance between the fan blade tips and the annular internal air path surface of the fan case.
In the case of small turbofan engines in particular, the clearance between fan blade tips and the fan case internal surface is often of a critical nature. Due to a high aerodynamic loading of the blades, the fan stage stall margin is sensitive to the tip clearance. Abnormal changes in tip clearance can adversely affect the engine thrust and surge margin, which must be avoided at all costs.
The fan of the turbo fan engine must comply with regulations intended to ensure safe operation of the turbofan engine in two critical conditions; firstly, on the ingestion of birds which strike the fan blading; and secondly, in the event of breakage of a fan blade. These two conditions are known generally as a "bird strike event" and a "blade off event".
In the prior art, a bird striking the fan generally results in an increase of tip clearance between the fan blade tips and the internal surface of the fan case. The soft abradable material bonded to the interior surface of the fan case is removed together with compressible material radially outward of the abradable material when the bird strike condition is encountered as follows. When an outboard bird is ingested into the forward fan area, the fan blades cut the bird into fragments and propel the fragments tangentially and axially rearwardly. The bird fragments are then expelled axially through the outward annular by-pass duct. However, in case of bird ingestion, some bird fragments are ingested into the engine core through the compressor and turbines.
Of particular interest to the present invention is the effect of a bird strike and resulting interaction of the fan blades with the fan case. The fan blades are deformed due to interaction. The axial and radial unbalanced loads are transmitted to the low power shaft, the supporting structure and the engine mounts. Therefore, the fan rotor will deflect radially outwardly and cut deeply into the compressible material and abradable material which lines the interior surface of the fan case.
Prior art fan cases for small engines are lined with approximately 0.100 to 0.300 inches of abradable material applied on the interior surface of an approximately 0.300 to 0.500 inch thick layer of compressible material. Twisted and deflected fan blades severely cut into these materials and lead to excessive fan tip clearances.
On a bird strike event, regulations require that the engine thrust decreases to no less than 75% of maximum engine thrust within 20 minutes after the bird strike. A number of engine components may be damaged due to the bird strike, however, the cumulative effect of various types of damage cannot reduce the total engine thrust by more than 25%. Bird strikes may deform the fan blades, damage the engine core, or damage compressor blades in addition to increasing the fan blade tip clearance dramatically. It has been found through experiment that excessive fan blade tip clearance can result in 7 to 9% of the thrust loss alone. Considering that regulations require no more than 25% engine thrust loss, it can be seen that excessive fan blade tip clearance after a bird strike is a significant cause of engine thrust loss.
In the case of small engines of approximately 24" fan diameter, it is common to include a layer of compressible material and abradable material of between 0.300 to 0.500 inches. The combined effect of blade deformation and fan rotor out of balance rotation can remove significant amounts of materials, especially toward the leading and trailing edges of the fan blades. It is not uncommon to encounter removal of 0.200 to 0.300 inches of such materials.
It has also been found that some fans are extremely sensitive to excessive tip clearance, and will stall. It can be seen therefore, that excessive tip clearance can lead to dangerous surge conditions on encountering bird strike events.
The prior art has provided means to limit tip clearance problems on bird strike by providing a hardwall fan case which comprises a rigid fan case shell parallel to the fan blade tips lined with a thin layer of abradable material to compensate for manufacturing tolerances and stretch of the blades in operation. On excessive movement of the fan blades during a bird strike event, the fan blade tips wear away the abradable material and directly contact the hardwall of the fan case. Fan rotors in general, are integrally bladed rotors. The fan case is lined with a layer of abradable material, since there is a concern that tight clearance during running of the engine will result in dynamic coincidence when the integrally bladed rotor rubs against the hardwall containment fan case before the rotor stabilizes around its own centre of rotation. Abradable material is therefore used to line a hardwall fan case to give sufficient clearance to stabilize the rotor around its own centre of rotation, and to limit tip clearance during bird strike events.
A significant disadvantage of a hardwall fan case however, is encountered on the second condition required of fan rotors namely, when a fan blade breaks off in the blade off condition. Standard tests are conducted on engine designs wherein an explosive charge is detonated to break off a fan blade during high speed operation, the fan case structure provides important protection for aircraft and passengers since the rapid rotation of the fan propels broken fan blade fragments radially at high speeds. The fan case therefore, is provided to contain any broken fan blade fragments within the engine itself, or to eject such fragments axially rearwardly through the by-pass duct.
The fan case in the prior art is an essential component to ensure that catastrophic accidents do not occur as a result of fan blades breaking off.
A hardwall fan case has a disadvantage resulting from the shape of the internal air path surface. The air path surface generally converges radially inwardly as the air taken into the engine increases in pressure and decreases in volume. The internal air path surfaces are tapered in such a manner that a broken fan blade fragment will bounce off the hardwall fan case and be redirected forwardly. This condition is unacceptable since further catastrophic damage may occur. The nacelle in the front of the engine will not contain the blade fragments propelled with high energy. Regulations require that any broken fan blade fragment be directed axially rearwardly to avoid further damage, or be contained within the fan case itself. Deflection of broken fan blade fragments forwardly, as well radial expulsion through the fan case itself are dangerous and unacceptable.
As a result, it has been common to provide a relatively heavy fan case shell which is lined with compressible material coated with abradable material. The compressible material acts to absorb the impact of the high velocity fan blade fragments. The rigid shells of the prior art fan cases are often tapered forwardly so that the radially expelled broken fan blade fragments will deflect rearwardly off the rigid fan case shell rather than forwardly. A forwardly inwardly tapered rigid fan case shell is commonly used for this purpose. However, providing the required thick layer of compressible material shaping the air path surface leads to unacceptable large fan tip clearances during a bird strike event as mentioned above. In the case of relatively large engines however, excessive fan tip clearance is less critical than in small engines.
Therefore, it can be seen that in the prior art there is a conflict between two competing conditions that must be accommodated by fan cases and fan blades. In the case of a bird strike, it is preferred that a hardwall fan case be provided to maintain the fan tip clearance within acceptable limits. However, in the case of fan blade breakage, it is preferred to line the fan case with a relatively soft compressible material that can absorb the impact with broken fan blade fragments and which has a tapered rigid shell surface that can deflect any broken fan blade fragments rearwardly. Due to the shape of the air path, in order to deflect broken fragments rearwardly, a hardwall fan case is generally inappropriate. The shape of the air pathway tapers inwardly as it progresses rearwardly through the engine, and the pressure of air increases with corresponding decrease in volume. By providing a hardwall fan case which follows the air path shape, any broken fan blade fragments will be deflected forwardly and impose the risk of unacceptable accidental damage to the aircraft for adjacent people and property. It is acceptable only to either retain the broken fragments within the fan case itself, or to eject broken fan blade fragments axially rearwardly.
Therefore, it is desirable to provide a fan case structure which can maintain fan tip clearance within acceptable limits after a bird strike event while simultaneously ensuring that any broken fan blade fragments are directed axially rearwardly, or retained within the fan case structure itself.
It is also desirable to provide such a fan case structure that will use existing materials and technology without requiring significant rework or re-certification of existing designs.